Macromolecular networks featuring cyanurate cross-links are highly valuable because of the unique combination of outstanding thermal resistance; favorable flame, smoke, and toxicity characteristics; relatively low cost for the level of performance achieved; mechanical toughness; and ease of processing. Liquid dicyanate ester monomers having viscosities below 1 Pa are recognized as affording some of the most affordable and convenient processes for the production of high-strength fiber-reinforced composites. Among these, Primaset® LECy (Lonza Ltd., Basel, Switzerland) is particularly well-known for its low viscosity and for providing a fully cured network with a glass transition temperature in excess of 250° C. Preparation of the liquid di(cyanate ester) resin principally found in Primaset®, 1,1-bis(4-cyaanthophenyl)ethane, is described in U.S. Pat. No. 5,284,968.
One disadvantage of cyanate ester monomers and oligomers for macromolecular network formation is that formation of the networks requires high temperature initiation of thermal polymerization via cyclotrimerization. The addition of metal-containing catalysts can effectively lower this temperature; however, care must be taken when introducing heat into these highly catalyzed systems. For example, excessive heat may cause runaway chemical reactions in which the heat produced by the reaction further accelerates the reaction in a self-reinforcing cycle.
Thus there exists a need for methods by which heat may be introduced into a cyanate ester resin, in a highly controlled manner, to affect thermal cure in a safe and effective manner.
One conventional solution to the controlled introduction of heat into a cyanate ester resin is the use of magnetic nanoparticles, such as those described in U.S. Pat. No. 8,565,892. Magnetic nanoparticles have unique capabilities in polymer nanocomposites, including, the ability to be positioned precisely in three-dimensional space by means of magnetic fields and without physically contacting the monomer or oligomer. Once positioned, significant amounts of heat may be generated by exposing the magnetic nanoparticles to high frequency, alternating electromagnetic fields. Therefore, magnetic nanoparticles have, at least in part, overcome some of the difficulties associated with introducing a controlled quantity of heat to precisely determined locations within a pre-determined region of a cyanate ester resin.
Yet, the conventional magnetic nanoparticles, which are comprised of ferromagnetic or superparamagnetic substances (such as cobalt or iron oxide), exhibit a number of shortcomings. For example, such conventional metallic, magnetic nanoparticles are susceptible to corrosion and may facilitate undesired, catalytic side reactions. One such undesired side reaction is the hydrolytic degradation of macromolecular cyanurate networks. Another such reaction is the decomposition of cyanurate networks at elevated temperatures. Networks containing magnetic nanoparticles may experience decomposition at significantly faster rates at temperatures where, in the absence of such nanoparticles, the decomposition takes place far more slowly, if it all.
Resultantly, there remains a need for methods of effectively isolating the magnetic nanoparticles from the macromolecular cyanurate network while retaining the advantageous ability of introducing a controlled quantity of heat to precisely determined locations within a pre-determined region of a cyanate ester resin sufficient to initiate cure of the resin.
For cyanate ester resins, the required cure temperatures range from about 120° C. to about 250° C., depending on the final properties of the network. These temperatures are significantly higher than the 60° C. that is required for many therapeutic applications. Accordingly, there also remains a need for methods of introducing a quantity of heat sufficient to raise the temperature to a level sufficient to initiate cure.